Relay system



Aug. 8, 1967 s. s. PRESSMAN RELAY SYSTEM Filed March 26, 1964 BNP) n H 3A o D v D a .A

im@ Ixo@ United States Patent M Electronics Corp., Floral Park, N.Y., acorporation of New York Filed Mar. 26, 1964, Ser. No. 354,859 8 Claims.(Cl. 317-124) The present invention relates generally to relay circuitsand more particularly to series resonant relay circuits which require nounilateral control devices for their operation, and which are operativeover a wide range of standard supply voltages with little or nodeleterious effects on operating characteristics.

Many applications exist for relay controls, wherein current to a load iscontrolled by a relay which changes its operative state in response toan activating element which senses a physical magnitude and whichchanges impedance as a consequence of such sensing. One typicalapplication of this type relates to the operation of relays in `responseto changes of light intensity, in which case the activating element maybe a photo-electric cell, or more specically a photo-electric cell ofthe photo-conductive type. Alternative systems involve activatingelements which are sensitive to temperature and which change resistanceupon change in temperature. More generally, the activating element maybe an impedance which is controlled by the value of a physical quantity.Throughout such applications of relay controls, the control circuit maybe required to operate in physical areas in which several differentsupply voltages are available. Previously, where relay controls wererequired to transfer power from a source to a load, it was necessary toutilize a different control circuit for each particular value ofstandard line voltage which might be available in the particular areawhere such use occurred.

The present invention concerns itself primarily with relay controlswherein uniform operating characteristics prevail over a wide range ofsupply voltages, i.e., those voltages which are to be supplied totheload, and wherein the activating element is of the photo-conductivetype, although as to the latter it is to be realized that in itsbroadest aspect it is not so limited but may be utilized in conjunctionwith any type of Variable impedance as an activating element. Forexample, capacitive elements are known which are light sensitive andinductive elements of the saturating type are known which changermpedance, or reactance, upon change of control current applied thereto,either of these types being among alternative elements which may be usedin the control circuit.

Activating elements usually are of a type which cannot carry heavycurrents, or, in the alternative, are of types which can be manufacturedmuch more economically if made to have extremely small current carryingcapacity. For example, the photo-conductor element is not normallycapable of carrying heavy currents when fabricated in convenient size,and a saturable inductance may be reduced in size as the currentrequired to be controlled by it is reduced. It is a feature of thepresent invention to provide a control circuit for a relay, controlbeing effected in response to an activating element which is notrequired to draw heavy current or to be of considerable size, and inwhich the relay is controlled directly by the activating element withoutrequiring the interposition of an amplifying device, such as a vacuumtube, a transistor, or the like. Thereby, the cost of a relay system maybe radically reduced, and its reliability under long term operatingconditions may be radically increased. A primary feature of the presentinvention resides in the capability of the control circuit to operateuniformly in transferring power from an A-C source to 3,335,331 PatentedAug. 8, 1967 a load over a relatively wide range of supply voltageswhich may be encountered in practical applications, Without need formodification of the control circuit structure. Thereby, where aconversion in voltage level of the A-C power supply is desired ornecessary, the relay control circuit embodiment of the present inventionmay be left unaltered in the system under conversion. Further, there maybe eliminated any requirement for maintaining a stock inventory ofvarious relay controls for use with a plurality of supply voltagelevels. Still further, the present relay control is relativelyuneflected by wide fluctuations in voltage from the A-C power supplyline, as commonly occur, for example, on electric utility supply lines.

Briefly describing a preferred embodiment of the invention, a relay coilhaving a saturable magnetic flux path is connected in series with acapacitor, with which it resonates or approximately resonates at theoperating frequency of the system, i.e., the A-C frequency of the powersupply. The present system is designed and intended primarily forenergization from power lines, i.e., at 60 c.p.s. The activatingelement, which in the preferred embodiment of the invention is aphoto-conductive cell, is connected preferably across the relay coil,although it may alternatively be connected across the capacitor. In theabsence of light the photo-conductive element has an extremely highresistance, and accordingly does not materially effect the seriescircuit consisting of the tuning capacitor and the relay coil. However,when the photoconductive element is illuminated, the Q of the seriescircuit, i.e., the ratio of inductive reactance to circuit resistance,is reduced, and this reduction may be radical. Accordingly, the totalvoltage across the relay coil, which is higher than line voltage underresonant or near resonant condition, may be reduced by reduction of theQ of the circuit to a comparatively low value, so that the relayswitches from an operating to a nonoperating condition. The effect ofthe photo-conductive cell on the Q of the circuit is enhanced so far ascurrent drawn by the relay coil is concerned, by the fact that thephotoconductive element shunts the relay coil and that the shuntingeffect is relatively slight when the photo-conductive cell isunilluminated but becomes considerable when the photo-conductive cell isilluminated. Nevertheless, the photo-conductive cell is not required topass relay current, so that a heavy relay may be operated by means ofthe present circuit in response to a photo-conductive cell which issmall and of low cost. Furthermore, the life of the photo-conductivecell is lengthened by the fact that it is not required to carry heavycurrents. As an alternative circuit arrangement, the photo-conductiveelement may be connected across the tuning capacitor instead of acrossthe relay coil, in which case illumination of the photo-conductiveelement changes the Q of the resonant circuit and thereby is able tocontrol operation of the relay. However, the total eect in the lattercase is smaller than when the photo-conductive element is connecteddirectly across the relay, because the shunting effect is lost.

In accordance with the present invention, the operating point on thesaturation curve of the voltage responsive relay is selected to be inthe saturation region to maintain operating coil voltage relativelyinvariant over a wide range of supply line voltage. For example, powersupply line voltage commonly encountered in practice may be either voltsor 240 volts, at 60 c.p.s. With the pre.- ferred embodiment of thecontrol circuit, briefly described above, connected across the line, anincrease of activating element resistance upon reduction of intensity oflight impinging thereon, results in an increase of resonant circuit Qand a consequent gradual increase in coil voltage. Thus relay coilpull-in begins at a particular point along the saturation curveirrespective of line voltage provided the latter is sufficiently high toproduce minimum operating coil voltage. This beginning condition ofpull-in is set near the bend, i.e., the knee, of the relay saturationcurve. At pull-in there is an abrupt change in relay coil impedance, aswill hereinafter be more fully explained, as flux density saturationbegins to occur in the magnetic path. For 120 volt line voltage,operation takes place largely in the saturation region where coilreactance, although decreased slightly from the value attained at thepreviously mentioned abrupt change, is still relatively high, and theresonant condition obtains. When 240 volt line voltage operation isrequired, the control circuit action is similar to that described aboveexcept that relay operation takes place even further into the relaysaturation region. At this operating point coil reactance is well belowthe value existing at 120 volt operation, the resonant circuit isdetuned, and thus the relay coil voltage is closely similar to thatwhich existed in the former case. Because there iS only slightdifference in coil voltage at either supply line voltage, and over abroad range which includes the two standard line voltages specified, itfollows that operating characteristics of the control circuit aremaintained substantially invariant. The importance of this feature isdernonstrated by the fact that no circuit modification is required forinstallation in power systems utilizing either supply line voltage. Inaddition, relay control circuit removal from the system is not requiredshould there be conversion from one line voltage to the other. Further,uniformity of operation with respect to illumination level is maintaineddespite rather rapid changes in impedance of the activating element, forexample a photo-conductive element, at low illumination levels.

It is accordingly a broad object of the present invention to provide arelay system having uniform operating characteristics over a wide rangeof supply voltage levels, in which the relay coil is contained in aseries resonant circuit, and in which a variable impedance is associatedwith the series resonant circuit in such fashion as to effect the Q ofthe resonant circuit, upon change in value of the impedance element,suiciently to change the operating state of the relay, and in which therelay coil operating Voltage varies only slightly irrespective of linevoltage level changes over a relatively wide range, by virtue of relayoperation in the saturation region.

It is a more specilic object of the present invention to provide a relaycircuit having relatively invariant operating characteristics intransferring power from a supply line to a load irrespective of changesof voltage level of the supply line across which the relay circuit isconnected, in which the relay coil is contained in a series resonantcircuit and in which a variable impedance is associated with the seriesresonant circuit in such manner as to affect the Q of the resonantcircuit upon change of value of the impedance element, and in which therelay coil reactance varies inversely with supply line voltage becauseof relay operation within the flux satuartion region so as to produce arelatively constant relay coil operating voltage over a wide range ofsupply line voltage levels.

A further object of the invention resides in the provision of a relaysystem for uniform operation irrespective of wide variation in supplyvoltage to the relay system, in

which the relay coil is located in series circuit with a tuningcapacitance, for series resonant operation, and in which a variableimpedance is associated with the series resonant circuit in such fashionas to affect the Q of the resonant circuit upon change of the impedancevalue of the impedance element, and in which the operating point of therelay is preselected to occur in the region of flux density saturationof the relay saturation curve, whereby increases in supply line voltagewill drive the relay further into its saturation lregion with resultantlowering of relay coil reactance and a consequent detuning of the seriesresonant circuit, maintaining relay coil operating voltage at asubstantially invariant level.

The above and still further objects, features, and attendant advantagesof the present invention will become apparent upon consideration of thefollowing detailed description of one specilic embodiment thereof,especially when taken in conjunction with the accompanying drawing,wherein:

FIGURE 1 is a schematic circuit diagram of a relay control circuit inaccordance with an embodiment of the present invention.

FIGURE 2 is a graph showing characteristics of a photocell of the typewhich may be used in the embodiment of FIGURE l;

FIGURE 3 is a graph showing relay coil characteristics exemplary of therelay coil of FIGURE l; and

FIGURE 4 is a graph indicative of conventional relay operatingcharacteristics of prior art relay circuits.

This application is related to the subject matter of copendingapplication Ser. No. 851,352, led Nov. 6, 1959, assigned to the sameassignee as the present invention.

Referring to FIGURE 1 of the accompanying drawing, the reference numeral10' denotes a source of line voltage across which a relay coil 12 andcapacitor 16 are connected in series circuit. In a preferred embodimentof the invention the line voltage source may have an A-C frequency of 60c.p.s. Relay coil 12 and capacitor 16 are of the proper characteristicsto form` a series resonant circuit at the frequency of the power line.Therefore, it follows that the total current llow to the relay coil 12will be greater than would be the case in the absence of the capacitor16, and that the voltage across the 4coil 12 will be ,greater than theline voltalge by a factor dependent upon the Q of the resonant circuit.The Q of the circuit is primarily determine-d by the resistance of relaycoil 12, and more accurately in terms of the ratio of coil inductivereactance to coil resistance at the operating frequency. Connecteddirectly across coil 12 is a photo-conductive cell 15. When thephoto-conductive element 15 is suiciently illuminated, as may -be notedfrom consideration of the photocell characteristics of FIGURE 2 whichare exemplary of photocells generally, the resistance across relay coil12 becomes relatively small and the series circuit Q is reduced. On theother hand, when the photocell is relatively unilluminated theresistance thereof is sufficiently high to result in very little effectupon the Q of the series resonant circuit, as determined by relay coil12.

That is, in the case of low photocell resistance, the phase of thevoltage across the capacitor 16 is no longer opposite the phase of thevoltage across relay coil 12, and moreover the relative magnitudes' ofthe two voltages becomeunequal, which may be described as reducing the Qof the resonant circuit. Therefore, in the operation of the relaycontrol circuit of FIGURE l, when the photocell is in a relativelyilluminated state, i.e. its resistance relatively low, the relay coilvoltage is not of sutlicient level to cause operation of the normallyopen contacts 14 of the relay. As the light intensity decreases, theseries resonant circuit Q increases and there is a correspondingincrease in the voltage across relay coil 12. Eventually, as thephotocell becomes unilluminated the coil voltage reaches a valuesuicient to cause operation of contacts 14. Thereupon the line voltageis connected directly `across the load which may, for example, be aplurality of lamps.

In order to provide damping eifects in the photo-conductive element pathto prevent relay operation should the photocell 15 become illuminatedfor a short instant of time, a separate element (not shown), for examplea ther-mister, which has the characteristics of gradual resistancechange with current flow heating may be connected in series circuit withthe photocell. Thus a delay would be provided to .prevent transienteffects as might occur, for example, if ay flash of light were projectedupon the photoconductive element.

As previously noted, in installing a system according to the presentinvention, the supply voltages encountered may differ according to thephysical environment, that is the geographical area, in which the relaysystem is to be utilized. Thus, for example, in a street lighting systemwherein the present invention may be used to provide street lightillumination control by selectively connecting the energizing linevoltage to the load on approach of night fall, a 12() volt or a 240 voltA-C line voltage may be provided. Operation of the relay system,however, is such that no modification of the relay control circuit isnecessary for use over a relatively wide range of line voltage including120 volts and 240 volts. This relatively invariant operation despitewide differences in supply voltage may be understood by reference toFIGURE 3, illustrating the operating characteristic curve of relay coilvoltage vs. current, which curve is largely dependent upon thecharacteristics of the magnetic flux path of the relay and as such isalso representative of relay magnetization curve. As a further aid tothe understanding of the relay operation, a second curve showing relaycoil impedance vs. coil current is Super-imposed on the graph of FIG-URE 3.

Referring now to FIGURE 3 as the resistance of photoconductive element15 increases with decreasing light intensity, for example asdarkness-falls, there is an accompanying gradual increase in voltageacross the relay coil, caused by the increase in the series resonantcircuit Q and the multiplying effects upon coil voltage thereof,irrespective of whether supply line voltage is 120 volts or 240 volts.Therefore in either case the relay armature 13 begins to pull in nearthe knee of the saturation curve, decreasing the air gap betweenarmature and relay core and thus decreasing the reluctance in themagnetic path. Because the greatest portion of magnetomotive forceproduced by the relay coil current was used in driving t-he flux acrossthe air gap, i.e., a high reluctance path, the rapid decrease in airga-p and its subsequent elimination results in a rapid decrease inreluctance, and increase in flux density, and a sharp rise in coilreactance as depicted by the relatively vertical line in the curveindicating coil impedance vs. coil current. At this point the magneticpath begins to saturate, that is flux density increases rapidly, asshown by the rapid.decrease in slope of the saturation curve, i.e., thecurve of coil voltage vs. coil current, and there is t-hus a consequentreduction in coil reactance, shown by the coil impedancecharacteristics.

The series circuit of relay coil 1,2 and capacitor 16 is tuned forresonance at the A-C frequency supply line Voltage, i.e., the impedanceof the relay coil 12 and capacitor 16 are counter balanced because ofthe phase difference in the relatively high voltages across each ofthese elements and specifically a difference in phase of approximately180. As previously noted, the relay coil voltage is dependent upon thevalue of Q `in terms of a multiplication factor times line voltage. Asmay be seen from FIGURE 3, relay operation at t-he 120 volt line levelis set to occur Within the saturation region of the magnetic flux path,that is, along the relatively decreasing slope portion of the coilvoltage vs. coil current curve.

-Upon reaching a .preselected operating coil voltage, relay operationoccurs in a reliable and positive manner.

Should the relay system be used in conjunction with a supply linevoltage of 240 volts there is no need for circuit modiiication becausethe relay coil voltage remains nearly the same as existed in theprevious case. In 240 volt operation, relay circuit action through coilpull-in remains substantially similar to that which occurred at 120 voltline voltage. However, at the 240 volt level the relay is driven evenfurther into the saturation region of the magnetic fiux path, as notedin the curve of coil voltage vs. coil current. Reliable relay operation,however, is preselected to occur in this further saturation condition.The series circuit of capacitor 16 and relay coil 12 is at this point nolonger resonant, i.e., is detuned, as a result of the decrease in therelay coil inductive reactance because of the increased saturation ofthe magnetic circuit. The major impedance in the series circuit isthus-the capacitive reactance, and a substantial voltage appears acrosscapacitor 16. However, coil voltage has increased only slightly from itsvalue as existed at 120 volts because of the decrease in coil reactance.

Thus, the doubling of supply line voltage presents no significant changein the operating characteristics of the relay system. Such uniformity ofoperation allows a single relay circuit to be stocked for use in powersupply systems, irrespective of the possibility of wide variations inline voltages. In a test of a relay control circuit constructed inaccordance with the foregoing description, substantially invariantoperation was obtained over a line voltage range from 110 volts to 277volts. Itis to be realized that reliable relay operation may bepreselected at any point within this range and not merely at theparticular voltages which have been specified. A suitable relay for useinthe relay control system described is the type designated RL-1800-4manufactured by Joseph Pollack Corporation, Aetna Motor ProductsCompany, of Boston, Mass.

The graph of FIGURE 4 is typical of the characteristics of conventionalrelays employed in relay control circuits of the prior art.Consideration of the curves of relay coil voltage vs. coil current andof relay coil impedance vs. coil current of such prior art controlcircuits will show that circuit operating characteristics vary directlywith line voltage changes. Except for the slight fluctuations occurringat relay coil pull-in, neither the slope of the relay coil Voltage curvenor that of the relay coil reactance curve varies significantly from aconstant value. Thus, for example, if a control circuit incorporating arelay having the characteristics depicted in FIGURE 4 for use in a 120volt power supply system were used in a 240 volt supply system, withoutextensive circuit modification, the large change in -relay coiloperating voltage at the higher line voltage would result in completelyunreliable operation. In the curve shown, a increase in line voltagerepresents a 93% increase in coil voltage. Further, such a relay for useat the higher voltage would be inoperative at the lower line voltage.Thus the value of relay operation in the flux density saturation regionof the relay magnetic path to provide uniformity of control circuitoperating characteristics irrespective of line voltage level changes, asis effected by the present invention, is readily appreciated. It is tobe further noted that where iiuctuations occur in the supply linevoltage the prior art relay control circuit would be incapable ofpositive and reliable operation but instead the relay contacts wouldchatter, i.e., make and break, according to the resulting variations inrelay coil voltage.

While the preferred embodiment of the present invention has beendescribed as utilizing the photo-conductive cell 15 as the activatingelement of the system, it will be appreciated that any variableresistance may be substituted therefor, exemplary elements being heatsensitive resistances, light sensitive capacitors, current sensitiveconductors, and the like. So long as the element substituted for thephoto-conductive element 15 can be varied between a very high value ofimpedance and a very low Value of impedance, the Q of the series circuitcomprising the capacitor 16 and the relay coil 12 can be radicallymodified, and the system will operate effectively. In any of thesecases, moreover, the activating element, with or without a transientdamping element, constitutes a shunt across the relay coil 12 andaccordingly has a control effect which is cumulative to the controleffect contributed by the series resonant circuit as such, so that thetwo cumulative effects can more positively control the voltage acrossrelay coil 12 than is the case for either effect alone. Further, the useof the saturated magnetic flux region of the relay operatingcharacteristics allows invariant operation over the previously notedwide range supply line voltages, assuring positive and reliable control,and as- 7 suring that the system will not chatter despite changes in thesupply voltage.

While there has been described and illustrated one specific embodimentof the present invention, it will be clear that variations of thedetails and structure which are specifically illustrated and describedmay be resorted to without departing from the true spirit and scope ofthe invention as defined in the appended claims.

What I claim is:

1. A control circuit for connecting an A-C power supply line to a load,said control circuit adapted for uniforrn operation over a relativelywide range of line voltages, comprising, in combination, a relay havingan induction coil and a saturable magnetic path, a capacitor connectedin series circuit with said coil to form a resonant circuit across thesupply line, an activating element having an impedance which varies inresponse to changes in ambient light intensity, means connecting saidactivating element in parallel circuit with one of said resonantelements so that said activating element controls the current passingthrough said coil to modify the Q of said resonant circuit when theambient light intensity on the activating element changes, said relayincluding means effective to connect said line to said load upon thecondition of a predetermined voltage across said relay coil, said relayhaving preselected operating points solely in the condition ofsaturation of the relay magnetic flux path whereby the operating voltageacross said relay coil is maintained substantially invariant over saidrelatively wide range of line voltages.

2. The combination according to claim 1 wherein said activating elementis a photo-conductive cell.

3. A relay system for uniform operation in transferring A-C power from asource to a load over a wide range of standard source voltages, saidsystem comprising a relay having at least a relay coil, a saturablemagnetic flux path, and a pair of contacts; a4 capacitive reactanceconnected in series circuit with said relay coil across said source,said series circuit having a resonant condition at the A-C frequency ofsaid source; said pair of contacts connected between said source andsaid load and adapted to transfer power therebetween upon operation ofsaid relay as defined by a predetermined voltage level across said relaycoil; said relay having a plurality of preselected operating pointswithin said range of standard source voltages; a first of said pluralityof preselected operating points at the lowest of said range of sourcevoltages at which said series circuit is in resonance and said magneticpath is saturated; a second of said plurality of preselected operatingpoints at the highest of said range yof said voltages, at which saidmagnetic path is further saturated whereby the inductive reactance ofsaid relay coil is gradually reduced in value from that at said firstoperating point and said series circuit is detuned; said magnetic pathbeing increasingly saturated as said source voltage is increased fromsaid lowest to said highest voltage level; said gradual reduction incoil reactance being effective to maintain said coil voltage atsubstantially said predetermined level throughout said range of sourcevoltages; said series circuit having a Q greater than unity; andvariable impedance means shunting one of said capacitive reactance andsaid relay coil for controllably modifying said Q value.

4. The combination according to claim 3 wherein said variable impedanceis a photo-conductive cell.

5. A relay control circuit for transferring A-C power from a source to aload, comprising, in combination, a relay having an induction coil and asaturable magnetic path; a capacitive reactance connected in seriescircuit with said coil to form a series resonant circuit across thesource; said series circuit having a resonant condition at the frequencyof said source; said series resonant circuit having a Q greater thanunity; circuit means, having an impedance which varies in response tochanges in ambient light intensity, connected in parallel circuit withone of said resonant elements for controllably modifying said Q value;said relay being effective to transfer said A-C power from the source tothe load upon the condition of a predetermined voltage across said coil;said transfer being preselected to occur at a condition of saturation ofsaid magnetic path whereby the inductive reactance of said coil may becontrollably modified to maintain said predetermined voltage relativelyinvariant with changes in supply voltage over a predetermined range.

6. The combination according to claim 5 wherein said circuit means is aphoto-conductive cell.

7. A control circuit for transferring power from an A-C source to aload, said control circuit adapted for uniform operation over a range ofline voltages from approximately volts to 275 volts supplied by saidsource, said control circuit comprising a voltage-operated relay, saidrelay having a coil, a saturable magnetic flux path, and at least onepair of contacts; a capacitive reactance connected in series circuitwith said relay coil across said source to form a series resonantcircuit at the A-C frequency of said source; said pair of contactsconnected between said source and said load, and adapted to transferpower therebetween upon operation of said relay as defined by apredetermined voltage level across said relay coil; said relay having arst preselected operating point, at which said series circuit is inresonance and said magnetic path is saturated, at the lowest of saidsource voltages; said series circuit having a Q of greater than unity;said relay having a second preselected operating point at the highest ofsaid source voltages, at which said magnetic path is further saturatedwhereby the inductive reactance of said relay coil is reduced in valueand said series circuit is detuned; said reduced coil reactance beingeffective to maintain said coil voltage at substantially saidpredetermined level at said lowest and said highest source voltages andin the range therebetween; and variable impedance means shunting one ofsaid capacitive reactance and said relay coil for controllably reducingsaid Q.

8. The combination according to claim 7 wherein said variable impedancemeans is a photo-conductive cell.

References Cited UNITED STATES PATENTS 4/1961 Mitchell et al. 317-125 X3/1963 Howell 307-117

1. A CONTROL CIRCUIT FOR CONNECTING AN A-C POWER SUPPLY LINE TO A LOAD, SAID CONTROL CIRCUIT ADAPTED FOR UNIFORM OPERATION OVER A RELATIVELY WIDE RANGE OF LINE VOLTAGES, COMPRISING, IN COMBINATION, A RELAY HAVING AN INDUCTION COIL AND A SATURABLE MAGNETIC PATH, A CAPACITOR CONNECTED IN SERIES CIRCUIT WITH SAID COIL TO FORM A RESONANT CIRCUIT ACROSS THE SUPPLY LINE, AN ACTIVATING ELEMENT HAVING AN IMPEDANCE WHICH VARIES IN RESPONSE TO CHANGES IN AMBIENT LIGHT INTENSITY, MEANS CONNECTING SAID ACTIVATING ELEMENT IN PARALLEL CIRCUIT WITH ONE OF SAID RESONANT ELEMENTS SO THAT SAID ACTIVATING ELEMENT CONTROLS THE CURRENT PASSING THROUGH SAID COIL TO MODIFY THE Q OF SAID RESONANT CIRCUIT WHEN THE AMBIENT LIGHT INTENSITY ON THE ACTIVATING ELEMENT CHANGES, SAID RELAY INCLUDING MEANS EFFECTIVE TO CONNECT SAID LINE TO SAID LOAD UPON THE CONDITION OF A PREDETERMINED VOLTAGE ACROSS SAID RELAY COIL, SAID RELAY HAVING PRESELECTED OPERATING POINTS SOLELY IN THE CONDITION OF SATURATION OF THE RELAY MAGNETIC FLUX PATH WHEREBY THE OPERATING VOLTAGE ACROSS SAID RELAY COIL IS MAINTAINED SUBSTANTIALLY INVARIANT OVER SAID RELATIVELY WIDE RANGE OF LINE VOLTAGES. 